Equipment

Mythbusting: Parfocal Photo Zooms

About twice a week I get a call from a photo-videographer wanting to know which photo zoom lenses in a certain focal length are parfocal (stays in focus while changing focal lengths). It’s understandable why they would ask; a superb photo zoom is 1/10th the cost of a good video zoom.

They get pretty mad when I tell them that none of them are. Often, they’ll tell me they know this lens is or that one is, because LensGuruGod1232 on their favorite forum shoots with this lens and says it is. They may even – if they’ve done some research – pull up an old article I wrote years ago listing some photo lenses that were parfocal and add that to the argument.

I have to tell them that article doesn’t count anymore. For one thing, parfocal is not an absolute definition. What is acceptable as parfocal to one person in one situation is unacceptable in another. Also, equipment has changed. A lens that might have appeared parfocal on a small sensor shooting standard definition video may be obviously not parfocal on a large modern sensor shooting 4k video. Finally, I’ve gotten older and wiser. Some things I wrote about years ago, I now realize are, um, well, less correct than I would have liked.

Today I’m going to demonstrate something I’ve known for a while, but that I haven’t written about. Sorry about that, but there’s lots of stuff I know that I haven’t written about. You see, I like to have written, but I don’t really like to write. I also know a lot of you really don’t like to read, so I’ll put the summary of the article right here at the top so you don’t have to read any further.

The ‘parfocalness’ of a photography zoom has significant copy-to-copy variation.

If you want to read further, I’ll give you a numeric demonstration and talk about why this is so. But the useful conclusion is in that one line above. So, the next time LensGuruGod1232 tells you the Wonderbar 24-70 f/2.8 lens is parfocal, remember that only means, at best, that some copies of it are. If your standards or sensors are different than his, maybe none of them are.

Let’s Focus Some Lenses

We took 10 copies of a good quality zoom lens (the Nikon 70-200 f/4 VR) each of which had been optically tested and found to be excellent. Then we set the focal length to 70mm, and carefully focused it on our optical bench. Why the optical bench? Because it gives us a numeric read out of the best focus position. You could do the same test if you have a reasonably accurate focus collimator and 10 copies of a lens handy.

We wrote down the focus position at 70mm and then, not touching anything else, zoomed the lens to 200mm. We then had the bench find the most accurate focus position at 200mm and made a table of the difference in focus position for each copy.

Olaf Optical Testing, 2016

The point is that the amount of focus change while zooming varies from copy to copy. Some shift hugely, some just a little. Most shift to the negative side, a couple to the positive.

Yes, there are some zooms where the average amount of shift is smaller than this. There are others where the average amount of shift is larger. But with every photography zoom we test, there is some copy-to-copy variation in how much focus shift occurs when you zoom from one end to the other.

I put an orange box around the area where we felt you could, at least arguably, call the lens parfocal. This isn’t a scientific box, it’s just where the focusing reticle, to me, still looked reasonably sharp.

If you want a more scientific comparison, we can compare to what we see if we do the same test using a cinema zoom. (We do this with cinema zooms pretty often, when we are checking to see if they remain parfocal.) If we average out the absolute change in best focus for the Nikon 70-200 f/4 VR in the graph above, when zooming from 70mm to 200mm it is 0.374mm. When we do the same test with a Zeiss 70-200 T2.9 lens the average change is 0.008mm.

If we look at the extremes, the photo zoom ranged from 0.742 to -0.273, a range of 1.12mm. The Zeiss 70-200 T2.9 cine zoom had a range of 0.05mm. Anything greater than that requires servicing.

Again, there are some photography zooms better than the Nikon as far as parfocal abilities. But there are none that we’ve seen that don’t have zoom variation. In other words, when someone says a photo zoom is parfocal, the truth is it’s not badly not parfocal, and even then there will be some copies that are more not parfocal than others.

Why Is This So?

The question really is, “How could it be otherwise”? Cinema zooms are made with a moving compensating group that has the specific purpose of maintaining focus while the lens is zoomed. This group is adjustable to make certain it compensates properly.

Photography zooms do not have anything like this compensating group. Additionally, as Aaron and I often see, when we adjust a photography zoom for the best optical performance we often change the ‘best focusing’ point at different focal lengths.

What’s the takeaway point? No photo zoom is going to be absolutely parfocal. Some may well be close enough for your purposes, but check the copy in your hands on the camera you’re going to use. Remember, when someone tells you a certain photo zoom lens is parfocal, that means their copy on their camera held to their standards.

Roger Cicala and Aaron Closz

Lensrentals.com

March, 2016

Author: Roger Cicala

I’m Roger and I am the founder of Lensrentals.com. Hailed as one of the optic nerds here, I enjoy shooting collimated light through 30X microscope objectives in my spare time. When I do take real pictures I like using something different: a Medium format, or Pentax K1, or a Sony RX1R.

Since STM lenses are ‘focus-by-wire’, the compensation can be done electronically. But you are correct that narrower apertures and shorter focal lengths allow the focus error introduced by zooming to be masked by the wider depth of field.

OnTheOtherHand

In this test it doesn’t matter which end of the zoom range you start, because you refocus at the other end.

The advice to focus zoomed in and then zoom out to take your picture is based on the lens not changing focus, which now is obviously it does, and as a saving grace, that when you zoom out, the wider angle, combined with the greater depth of field will mask the fact that the lens is not in perfect focus.

Michael Newsom

I think the message from phone cameras and almost every other gadget is ‘software rules.’

ChrisInCalif

Why test the lenses by focusing at 70mm then zooming out to 200mm? This seems backwards. Why not focus at
the higher focal length then zoom back down (because it is easier to see what you’re focusing on at highest focal length)?

Dennis L Sørensen

Sony E PZ 18-110mm f/4 is a fly by wire parfocal lens. There is a small servo that adjusts the focus as you zoom. It’s is programmed into the lens, even when you have manual focus on the camera. so as you zoom, it know when it “should” be in focus.

balazer

I have boatloads of TV and photographic zoom lenses, and based on the many tests I’ve done in my long quest for parfocalness I can tell you that a good chunk of the photographic zoom lenses are quite parfocal, even in 4k. The best of them are just as good as a modern TV zoom lens.

But you’ll only know how parfocal a lens really is if you adjust the back focus. If you didn’t (and it sounds like you didn’t), then your conclusions are invalid. A lens is almost never parfocal if you just stick it on the camera. The back focus must be adjusted, either with a built-in back focus adjustment, or by changing the flange focal distance, with a mount adjustment, shims, a helicoid adapter, etc. Photographic zoom lenses rarely make those adjustments easy, seeing as how parfocalness is not a priority for photo shooting. Find a way to make the adjustment, and then you discover just how parfocal the lens is. The camera’s sensor-to-flange distance can vary too, so the adjustment really needs to be done for a particular camera. Even the temperature can change back focus.

Yes, the degree of parfocalness in zoom lenses varies from model to
model and from copy to copy. But that’s not saying anything. Every
aspect of lens performance varies.

A lens can be less than perfectly parfocal in a lot of different ways. The cams could have too much play, causing the focus to shift every time the zoom direction changes. Or the cam path might be imprecise, causing the focus to shift at one end or the other of the zoom range, or in the middle. Some lenses are quite parfocal over a good 80% of the zoom range, and lose it at the very end. A lens might be acceptably parfocal at one aperture setting, but not at a larger one. Stopping down by just one stop can make a huge difference in some lenses. Some zoom lenses are parfocal at one focus distance, but not at another. Some lenses are parfocal when held horizontally, but not when tilted up or down.

If a lens is even remotely close to being parfocal, then it has a zoom compensating group. That makes it a zoom lens by definition, and zoom lenses are parfocal by definition. If a lens doesn’t have compensating group, it’s called a varifocal lens, and it is wickedly far from being parfocal. Varifocal lenses don’t even have a single focus distance scale marked on the lens. The focus distance scale will depend on the focal length, if it even has a distance scale marked.

There is a third category: lenses that are parfocal, but only electronically. They’ll lose focus momentarily when you zoom in or out, but then snap back into focus as soon as the focus motor has a chance to catch up with whatever the zoom encoder is saying. A lot of mirrorless lenses fall in this category.

TV and cinema zooms are more parfocal than photographic zooms (on average) for two reasons. One is that they were designed to be, with very tight tolerances in the zoom cams. The other is that the flange focal distance has been adjusted very carefully.